The present invention relates to an apparatus and method for obtaining mechanical energy directly from the expenditure of the chemical energy of fuel burned in a combustion chamber that is an integral part of the apparatus, and more particularly to an internal-combustion engine.
Internal-Combustion Engine is any type of machine that obtains mechanical energy directly from the expenditure of the chemical energy of fuel burned in a combustion chamber that is an integral part of the engine.
In 1873 Brayton, an American, developed an engine, which had the unique features of constant-pressure combustion and complete expansion. One cylinder was used to compress air or the combustible mixture. Another cylinder was used as a working cylinder and was large enough to obtain complete expansion to atmospheric pressure. The compressor discharged the mixture into a receiver, and the mixture flowed from the receiver to the engine, being ignited and burned at constant pressure as it entered the engine. An ignition flame was supported by a mixture by-pass, and a flame-suppression grid prevented the flame from flashing back into the mixture receiver.
The Brayton engine could not compete with the Otto-cycle engine because of high heat and mechanical-friction losses, and it was abandoned when the Otto-engine was introduced in the United States. Although the Brayton process was abandoned for the piston engine, it is used for gas-turbine engine process.
Four principal types of internal-combustion engines are in general use: the Otto-cycle engine, the Diesel engine, the rotary engine, and the gas turbine.
The Otto-cycle engine, named after its inventor, the German technician Nikolaus August Otto, was first built in 1876 and is the familiar gasoline engine used in automobiles and airplanes.
The Diesel engine, (U.S. Pat. No. 542,846, granted on Jul. 16, 1895) named after the French-born German engineer Rudolf Christian Karl Diesel, operates on a different principle and usually uses oil as a fuel. It is employed in electric-generating and marine-power plants, in trucks and buses, and in some automobiles. Both Otto-cycle and Diesel engines are manufactured in two-stroke and four-stroke cycle models.
The essential parts of Otto-cycle and Diesel engines are the same. The combustion chamber consists of a cylinder, usually fixed, that is closed at one end and in which a close-fitting piston slides. The in-and-out motion of the piston varies the volume of the chamber between the inner face of the piston and the closed end of the cylinder. The outer face of the piston is attached to a crankshaft by a connecting rod. The crankshaft transforms the reciprocating motion of the piston into rotary motion. In multi-cylindered engines the crankshaft has one offset portion, called a crankpin, for each connecting rod, so that the power from each cylinder is applied to the crankshaft at the appropriate point in its rotation. Crankshafts have heavy flywheels and counterweights, which by their inertia minimize irregularity in the motion of the shaft. An engine may have from 1 to as many as 28 cylinders.
The fuel supply system of an internal-combustion engine consists of a tank, a fuel-pump, and a device for vaporizing or atomizing the liquid fuel. In Otto-cycle engines this device is either a carburetor or, more recently, a fuel-injection system. In most engines with a carburetor, vaporized fuel is conveyed to the cylinders through a branched pipe called the intake manifold and, in many engines, a similar exhaust manifold is provided to carry off the gases produced by combustion. The fuel is admitted to each cylinder and the waste gases exhausted through mechanically operated poppet valves or sleeve valves. The valves are normally held closed by the pressure of springs and are opened at the proper time during the operating cycle by cams on a rotating camshaft that is geared to the crankshaft. By the 1980s more sophisticated fuel-injection systems, also used in Diesel engines, had largely replaced this traditional method of supplying the proper mix of air and fuel. In engines with fuel injection, a mechanically or electronically controlled monitoring system injects the appropriate amount of gas directly into the cylinder or inlet valve at the appropriate time. The gas vaporizes as it enters the cylinder. This system is more fuel-efficient than the carburetor and produces less pollution.
In all engines some means of igniting the fuel in the cylinder must be provided. For example, the ignition system of Otto-cycle engines described below consists of a source of low-voltage, direct current electricity that is connected to the primary of a transformer called an ignition coil. The current is interrupted many times a second by an automatic switch called the timer. The pulsations of the current in the primary induce a pulsating, high-voltage current in the secondary. The high-voltage current is led to each cylinder in turn by a rotary switch called the distributor. The actual ignition device is the spark plug, an insulated conductor set in the wall or top of each cylinder. At the inner end of the spark plug is a small gap between two wires. The high-voltage current arcs across this gap yielding the spark that ignites the fuel mixture in the cylinder.
Because of the heat of combustion, all engines must be equipped with some type of cooling system. Some aircraft and automobile engines, small stationary engines, and outboard motors for boats are cooled by air. In this system the outside surfaces of the cylinder are shaped in a series of radiating fins with a large area of metal to radiate heat from the cylinder. Other engines are water-cooled and have their cylinders enclosed in an external water jacket. In automobiles, water is circulated through the jacket by means of a water pump and cooled by passing through the finned coils of a radiator. Some automobile engines are also air-cooled, and in marine engines seawater is used for cooling.
Unlike steam engines and turbines, internal-combustion engines develop no torque when starting, and therefore provision must be made for turning the crankshaft so that the cycle of operation can begin. Automobile engines are normally started by means of an electric motor or starter that is geared to the crankshaft with a clutch that automatically disengages the motor after the engine has started. Small engines are sometimes started manually by turning the crankshaft with a crank or by pulling a rope wound several times around the flywheel. Methods of starting large engines include the inertia starter, which consists of a flywheel that is rotated by hand or by means of an electric motor until its kinetic energy is sufficient to turn the crankshaft, and the explosive starter, which employs the explosion of a blank cartridge to drive a turbine wheel that is coupled to the engine. The inertia and explosive starters are chiefly used to start airplane engines.
Otto-Cycle Engines
The ordinary Otto-cycle engine is a four-stroke engine; that is, in a complete power cycle, its pistons make four strokes, two toward the head (closed head) of the cylinder and two away from the head. During the first stroke of the cycle, the piston moves away from the cylinder head while simultaneously the intake valve is opened. The motion of the piston during this stroke sucks a quantity of a fuel and air mixture into the combustion chamber. During the next stroke, the piston moves toward the cylinder head and compresses the fuel mixture in the combustion chamber. At the moment when the piston reaches the end of this stroke and the volume of the combustion chamber is at a minimum, the fuel mixture is ignited by the spark plug and burns, expanding and exerting a pressure on the piston, which is then driven away from the cylinder head in the third stroke. During the final stroke, the exhaust valve is opened and the piston moves toward the cylinder head, driving the exhaust gases out of the combustion chamber and leaving the cylinder ready to repeat the cycle.
The efficiency of a modern Otto-cycle engine is limited by a number of factors, including losses by cooling and by friction. In general, the efficiency of such engines is determined by the compression ratio of the engine. The compression ratio (the ratio between the maximum and minimum volumes of the combustion chamber) is usually about 8 to 1 or 10 to 1 in most modern Otto-cycle engines. Higher compression ratios, up to about 15 to 1, with a resulting increase of efficiency, are possible with the use of high-octane antiknock fuels. The efficiencies of good modern Otto-cycle engines range between 25 and 30 percent—in other words, only this percentage of the heat energy of the fuel is transformed into mechanical energy.
Diesel Engines
Theoretically, the Diesel cycle differs from the Otto cycle in that combustion takes place at constant volume rather than at constant pressure. Most Diesels are also four-stroke engines but they operate differently than the four-stroke Otto-cycle engines. The first, or suction, stroke draws air, but no fuel, into the combustion chamber through an intake valve. On the second, or compression, stroke the air is compressed to a small fraction of its former volume and is heated to approximately 440° C. (approximately 820° F.) by this compression. At the end of the compression stroke, vaporized fuel is injected into the combustion chamber and burns instantly because of the high temperature of the air in the chamber. Some Diesels have auxiliary electrical ignition systems to ignite the fuel when the engine starts and until it warms up. This combustion drives the piston back on the third, or power, stroke of the cycle. The fourth stroke, as in the Otto-cycle engine, is an exhaust stroke.
The efficiency of the Diesel engine, which is in general governed by the same factors that control the efficiency of Otto-cycle engines, is inherently greater than that of any Otto-cycle engine and in actual engines today is slightly more than 40 percent. Diesels are, in general, slow-speed engines with crankshaft speeds of 100 to 750 revolutions per minute (rpm) as compared to 2500 to 5000 rpm for typical Otto-cycle engines. Some types of Diesel, however, have speeds up to 2000 rpm and even higher. Because Diesels use compression ratios of 14 or more to 1, they are generally more heavily built than Otto-cycle engines, but this disadvantage is counterbalanced by their greater efficiency and the fact that they can be operated on less expensive fuel oils.
Two-Stroke Engines
By suitable design it is possible to operate an Otto-cycle or Diesel as a two-stroke or two-cycle engine with a power stroke every other stroke of the piston instead of once every four strokes. The efficiency of such engines is less than that of four-stroke engines, and therefore the power of a two-stroke engine is always less than half that of a four-stroke engine of comparable size.
The general principle of the two-stroke engine is to shorten the periods in which fuel is introduced to the combustion chamber and in which the spent gases are exhausted to a small fraction of the duration of a stroke instead of allowing each of these operations to occupy a full stroke. In the simplest type of two-stroke engine, sleeve valves or ports (openings in the cylinder wall that are uncovered by the piston at the end of its outward travel) replace the poppet valves. In the two-stroke cycle, the fuel mixture or air is introduced through the intake port when the piston is fully withdrawn from the cylinder. The compression stroke follows, and the charge is ignited when the piston reaches the end of this stroke. The piston then moves outward on the power stroke, uncovering the exhaust port and permitting the gases to escape from the combustion chamber.
Rotary Engine
In the 1950s the German engineer Felix Wankel developed an internal-combustion engine of a radically new design, in which a three-cornered rotor turning in a roughly oval chamber replaces the piston and cylinder. The fuel-air mixture is drawn in through an intake port and trapped between one face of the turning rotor and the wall of the oval chamber. The turning of the rotor compresses the mixture, which is ignited by a spark plug. The exhaust gases are then expelled through an exhaust port through the action of the turning rotor. The cycle takes place alternately at each face of the rotor, giving three power strokes for each turn of the rotor. Because of the Wankel engine's compact size and consequent lesser weight as compared with the piston engine, it appeared to be an important option for automobiles. In addition, its mechanical simplicity provided low manufacturing costs, its cooling requirements were low and its low center of gravity made it safer to drive. A line of Wankel-engine cars was produced in Japan in the early 1970s, and several United States automobile manufacturers researched the idea as well. However, production of the Wankel engine was discontinued as a result of its poor fuel economy and its high pollutant emissions.
Stratified Charge Engine
A modification of the conventional spark-ignition piston engine, the stratified charge engine is designed to reduce emissions without the need for an exhaust-gas re-circulation system or catalytic converter. Its key feature is a dual combustion chamber for each cylinder with a pre-chamber that receives a rich fuel-air mixture while the main chamber is charged with a very lean mixture. The spark ignites the rich mixture that in turn ignites the lean main mixture. The resulting peak temperature is low enough to inhibit the formation of nitrogen oxides, and the mean temperature is sufficiently high to limit emissions of carbon monoxide and hydrocarbon.
Research on modifications of conventional engines as well as alternatives to conventional engines continues. Some of these options include a modified version of the two-stroke engine, the twin engine (a combination of an internal-combustion engine and an electric engine), and the Stirling engine.
Stirling Engine
Stirling engine is a type of engine that derives mechanical power from the expansion of a confined gas at a high temperature. The engine was patented in 1816 by the Scottish clergyman Robert Stirling and was used as a small power source in many industries during the 19th and early 20th centuries. The need for automobile engines with low emission of toxic gases has revived interest in the Stirling engine, and prototypes have been built with up to 500 hp and with efficiencies of 30 to 45 percent. Common internal-combustion engines have efficiencies in the range of 20 to 30 percent.
The cycle that provides the work is called the Stirling cycle. It consists in its simplest form of the compression of a fixed amount of so-called working gas (hydrogen or helium) in a cool chamber. This cool compressed gas is transferred to a hot chamber, which is heated by an external burner, where the gas expands and drives a piston that delivers the work. The expanded hot gas is then cooled and returned to the cold chamber, and the cycle begins again. The engine is able to transform heat into work because the expansion of the gas at high temperature delivers more work than is required to compress the same amount of gas at low temperature.
An external continuous burner that can operate on gasoline, alcohol, natural gas, propane, or butane, provides the heat for the expansion chamber, and the exhaust generated has very low free carbon and toxic gas levels. The Stirling engine runs smoothly because pressure variations in the compression and expansion chambers are sinusoidal, that is, relatively gradual, rather than explosive as in internal-combustion cycles. The necessity of rapid removal of heat from the hot working gas requires a large radiator, which makes this type of engine less suited to small automobiles.
The Scuderi Split-Cycle Engine
The Scuderi Split-Cycle Engine presently under development divides (or splits) the four strokes of the Otto cycle over a paired combination of one compression cylinder and one power (or expansion) cylinder, operating in principle like the Brayton engine in 1873. These two cylinders perform their respective functions once per crankshaft revolution.
The concept is shown in FIG. 1 where an intake charge is drawn into the compression cylinder 10 through an intake gas passage way and through typical poppet-style valves. Gas is compressed in the compression cylinder 10 and transferred to a compressed gas accumulator 14 and/or the power cylinder 12 through a crossover gas passage, which acts as the intake port for the power cylinder 12. The crossover gas passage includes a set of uniquely timed valves, which maintain a pre-charged pressure in the compressed gas accumulator 14 through all four strokes of the cycle. A check valve is used to prevent reverse flow from the crossover gas passage to the compression cylinder 10. Likewise a poppet-style valve prevents reverse flow from the power cylinder 12 to the crossover passage during the power and exhaust strokes.
Shortly after the piston in the power cylinder 12 reaches its top dead center position, the gas is quickly transferred to the power cylinder 12 and fired (or combusted) to produce the power stroke. The exhaust gases are pumped out of the power cylinder 12 during its return exhaust stroke through a typical poppet valve to the exhaust passage way.